Table Of ContentMICROF%CH
REFERENCE
LlBRaRY
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in Asia
a4fs_i.saTida'l
Wave?
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WHAT IS A TIDAL WAVE?
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NTERTECT
WHAT IS A TIDAL WAVE?
1.
What is a tidal
wave?
A tidal
wave is a rapid
rise
in the level
of the sea which
is caused
by a cyclone.
A tidal
wave is not actually
a wave; it
is a large
mass of
water which rises
above the normal
level
of the sea and is pushed along by
the cyclone,
moving in the same direction
as the cyclone
system.
This
movement creates
a front
or leading
edge which may be many miles
&ms.)
across
and as high as 65 feet
(20 m.)
The high water
behind
the leading
edge may extend backward
from the front
for many miles
(kms.)
Tidal
waves are more properly
called
storm surges.
2.
What causes a tidal
wave?
A tidal
wave is created
by a number of factors
associated
with
a
cyclone.
First,
the air
pressure
in the center
of a cyclone
is very low.
This release
of pressure
on the sutiace
of the sea allows
the sea to expand
and rise
upward in a hump.
Second; as the fast
winds 0f.a
cyclone
move
over the water,
they pile
the water
up as the cyclone
moves toward
the
coast.
Large waves are pushed against
the shore but cannot retreat
from
the coastline.rapidly
because the winds slow their
return
into
the sea.
This problem
is made more severe
if
the sea bed is shallow.
By the time
the center
(or eye) of the cyclone
arrives,
the large
mass of water
raised
above the normal
level
of the.sea
by the low pressure
at the eye rides
over the water which has been piled
up by cyclonic
winds and moves inland
across
the shore in one rapid
surge.
3.
Do all
cyclones
have tidal
waves?
I Every cyclone
has a tidal
wave.
Many are small,
but a few are very
large.
Many different
factors
determine
how big a tidal
wave is,
how far
it
moves inland,
and how much damage it
does.
1
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*
4.
What does a tidal
wave look like?
The drawing
below shows what a typical
tidal
wave looks
like
in
cross-section.
While the leading
edge of a tidal
wave appears
to someone
standing
in its
path as a wave,
a rapid
rise
in sea level,
it
must be remembered that
it
is in fact
and everything
in its
path will
be submerged
for a long period
of time after
the leading
edge has passed.
2
5.
In what direction
do tidal
waves move?
A tidal
wave moves in the same direction
that
the cycione
is moving,
not in the direction
of the wind.
A tidal
wave is normally
pushed along
in
front
of a cyclone.
6.
How fast
does a tidal
wave move?
A tidal
wave moves at the same speed as the cyclone
itself.
If
the
cyclone
is moving forward
at a speed of 12 miles
per hour
(20 kph),
the
tidal
wave moves at a speed.of
12 mph (20 kph).
If
the cyclone
is moving
at 60 mph (100 kph),
the tidal
wave moves at 60 mph (100 kph).
The tidal
wave which
struck
Andhra Pradesh was moving at approximately
20 mph (32 kph)
and this
is the estimated
speed of the tidal
wave.
7.
How high
is a tidal
wave?
There are four
factors
which determine
the height
of a tidal
wave:
4.
A.
The air
pressure
at the center
of the storm:
B.
The speed of the cyclone;
C.
The speed of the winds in the cyclone;
D.
The height
of a normal
tide.
Of these factors,
the last
one is one of the mcst important
to consider.
For example,
the low pressure
at the center
CT:: the storm may raise
the.ievel
of the sea 20 feet
(6 m).
The winds which pile
up the water
usually
add
another
five
feet
(1.5 m).
The speed of the cyclone
usually
adds another
5-10 feet
(1.5
- 3 m).
If
the tidal
wave arrives
at the coastline
at the
low tide
period,
then the height
of the wave will
be reduced by an amount
equal to the difference
between normal sea level
and the low tide.
In other
words,
if
low tide
is ten feet
(3 m) below sea level,
then the height
of the
tidal
wave will
be reduced
by ten feet
(3 m).
However,
if
a tidal
wave arrives
at high
tide,
then the height
of the
'tidal
wave will
be increased
by the height
of the tide
above normal sea level.
Thus, if
the normal high tide
is ten feet
(3 m) above sea level,
that
ten
feet
(3 m) will
be added to the height
of the tidal
wave.
The tidal
wave
which struck
Andhra Pradesh arrived
almost
at the time of the highest
tides,
8.
What is the highest
tidal
wave that
could
occur
in Andhra Pradesh?
Theoretically
, the highest
tidal
wave which could occur
in Andhra Pradesh
would be approximately
45-50 feet
(13-16 m).
Add to this
the height
of the
accumulated
wind and wave action,
and assume that
the surge arrives
during
the high
tide
period.
shoreline.
A tidal
surge of over 65 feet
(20 m) could
cross
the
Tidal
waves of this
height
have been recorded
in other
areas of
the Bay of Bengal.
The tidal
wave which struck
Bangladesh
in November 1970
was estimated
to have been 65 feet
(20 m) high when it
crossed
the shoreline.
w.
3
9.
~~~ fag inland
does a tidal
wave
move?
A tidal
wave will
move inland
until
it
reaches
a point
where the ground
is as high above sea level
as the top of the tidal
surge.
In other
words,
if
a tidal
wave is 20 feet
(6 m) high,
it will
continue
to move inland
until
it
reaches
a point
where the land
is 20 feet
(6 m) above sea level.
Before
the tidal
wave reaches
this
point,
however,
the leading
edge will
have crashed
and the deadly
impact of the leading
edge will
be lessened
from that
Point
of
crash.
The wave crashes
when it
reaches
an area where the water
depth becomes
too shallow
for
the wave-like
motion
of the leading
edge to continue.
When
the wave crashes,
the water which
continues
inland
will
be extremely
violent,
moving
up and down at the surface
creating
smaller,
rapid-moving
waves which
criss-cross
each other,
and carrying
much underwater
turbulence
with
it.
The
momentum of the water
may carry'the
surge slightly
beyond the contour
level
of the land which
is equal
to the original
height
of the wave.
Thus a number
of areas beyond that
point
can be inundated
with
water,
but the water moving
in these areas will
not have the force
or energy of the original
tidal
surge.
10.
How long will
a tidal
wave remain in the area?
i-
Once a tidal
wave has stopped
moving forward,
the water will
begin
to
retreat
to the sea.
however,
on the surface
of the water,
new waves may be
created
by the action
of the high winds associated
with
a cyclone.
All
the
water
will
not leave until
the eye of the cyclone
has passed and the winds
begin
to subside
as the cyclone
moves inland.
The actual
tidal
wave itself
may inundate
an area for as long as 45 minutes.
It
shouId be remembered
that
the torrential
rains
which accompany a cyclone
themselves
contribute
to the severe flooding
problems
which occur after
the actual
tidal
wave has
receded
into
the sea.
11.
What is the farthest
distance
that
a tidal
wave has been known to move
inland?
Tidal
waves have been known to go as far
inland
as 65 miles
(105 kms).
.12.
What is the effect
of tidal
waves on houses?
A tidal
wave can damage or destroy
a house in many different
ways.
First,
the speed of the tidal
wave places
great
stress
and force
on the walls,
just
E,y the great
momentum of the water.
Second, the currents
and turbulence
asso-
&at&
with
a tidal
wave destroy
a structure
by eroding
the foundation.
-
Third,
the debris
carried
by a tidal
wave (such as uprooted
trees,
fences,
Parts
of broken houses)
will
act as battering
rams which cause further
damage
and destruction
to the structures.
Fourth,
other
damage may be caused by the
sand and gravel
carried
by the fast-moving
currents
at the bottom of a tidal
wave, having
the effect
of "sand papering"
the foundations
of buildings.
Fifth,
damage can be caused by water
lifting
parts
of a structure
UP
to
float
on top of the wave.
This
is
know as the "flotation
effect"
and it
can happen
.
s.
,
4
even to buildings
made of brick
and cement.
It
is caused by the rapid
rise
of the water
outside
a building,
which
creates
such pressure
differences
that
the building
attempts
to float.
Once the walls
have left
the foundation
and
water
enters
the structure,
the building
will
collapse.
13.
How can a person protect
himself
from a tidal
wave?
There is no means of erecting
a safe shelter
to protect
a person from
a tidal
wave.
The only way to protect
your life
is to evacuate
the area
as soon as the threat
of a tidal
wave is known.
Every cyclone
has a tidal
wave; and as pointed
out in question
#7, the height
of a tidal
wave depends
on many factors.
However,
there
are certain
areas that
will
always be hit
by tidal
waves.
These are areas along
the river
deltas
and regions
which
have particularly
large,
low-lying
areas along
the coast with
adjacent
sea
beds which are very shallow.
If you live
near one of these areas,
the only
real
protection
you have is to leave
the area and move to high
ground.
There are no shelters
which
can be built
to be absolutely
safe in a tidal
wave.
Even if
a shelter
could withstand
the force
of the turbulent
water,
it
could
still
be damaged or destroyed
by debris
propelled
by the tidal
surge.
Even if
the building
survived,
the people
inside
might well
be
drowned.
14.
How can the risk
of damage or destruction
to property
be decreased?
The only
type of structure
which has any chance.of
surviving
a tidal
wave is one built
on stilts.
Even so, these structures
must have a floor
which is above the height
of a tidal
wave;
exposed to the tidal
wave and water
forces;
they must have a minimum of area
and they must be b-raced and
anchored
properly.
This type of structure,
however,
is very impractical
for
the area of Andhra Pradesh and is very expensive
to build.
Furthermore,
the survivability
of this
type of structure
is dependent
on its
being above
the height
of the tidal
surge;
as the wave may be as high as 65 feet
(20 m),
the chance that
these structures
will
survive
is still
only minimal.
For houses that
are located
on the coast,
there
is no way of making
sure that
they will
survive
a tidal
wave.
However,
for
those located
several
-miles
(km)
inland,
there
are a number of steps
that
can be taken to reduce
the risk
of damage:
A.
Clear
away any possible
sources
of debris
which
crj,.ld be
lifted
by a tidal
wave and thrown
into
the side
a house.
When a tidal
wave is expected,
move all
loobe o
zts (such
as tools,
loose wood, bamboo, carts,
etc.)
intl
or put them behind
the building
so that
they L
building
not be
lifted
up and thrown
into
the side.
B.
When constructing
a house,
as possible.
raise
the floor
level
a's high
Earth
can be taken from nearby paddies
and
drainage
canals
to use in raising
the plinth
area of a
structure.
-The higher
a building
can be built,
the better.
-,
5
C.
D.
E.
F.
_-F-c-
.---
Add as much bracing
as possible
in the walls
for
support,
especialiy
at the corners.
Follow
all
the rules
for building
better,
wind resistant
structures,
paying
special
attention
to the foundation,
and
making sure that
all
the poles
put in the ground are deep
and properly
treated.
Plant
suitable
shrubs
and trees
to protect
the house by
serving
as a barrier
against
debris
propelled
by the tidal
wave.
Most importantly,
he sure to evacuate
the area as soon as
there
is any chance of a tidal
wave occurring.
Remember,
it will
be too late
when you see the water
coming, as the
combined wind and wave forces
will
knock you over before
you can get to high ground and shelter.
15.
Are trees
and forests
good protection
from tidal
waves?
ah .
Trees and forests
are good protection
from tidal
waves as long as the
trees
which are planted
have deep root
systems.
However,
the trees
which
are normally
planted
along 'the coast
and which can grow in these areas do
not usually
have deep root
systems and are therefore
likely
to be ripped
loose
by the tidal
wave and thrown
into
buildings
further
inland.
Trees
and shrubs are good protection
only
if
they are pl,anted
several
miles
inland
from the coast.
16.
Are seawalls
and bunds good protection
against
tidal
waves?
A seawall
serves
two purposes.
The first
is to prctect
the area behind
the seawall
from flooding.
One seawall
along
the edge of the coast will
serve as protection
only against
very small. tidal
surges
(those which are
3-5 feet/l-l
l/2
m. in height)
or larger
tidal
waves which occur during
a low
tide.
They will
not provide
much protection
against
iarger
tidal
waves
which occur during
a high
tide.
The second purpose
of a seawall
is to break up the force
and energy of
a tidal
wave.
The effectiveness
of a seawall
for
this
purpose
again depends
on the height
of the wave and on the volume of water
which is being
propelled
'
in the total
surge.
Often a seawall
can force
the leading
waves of a surge
to crash as they cross
the seawall;
thus much of the turbulence
associated
with
the tidal
wave can be partially
reduced.
will
offer
only minimal
protection.
Overall,
however,
one seawall
A series
of seawalls
or bunds which are constructed
in parallel
lines
several
miles
apart
can be effective
in reducing
the total
amount of area
which
is inundated.
_
If designed
properly,
'they can serve to sloti down and
eventually
stop a tidal
wave,
wave moves 'iinland.
thus reducing
the distance
to which a tidal
A good, elevated
road system can be designed
in such a
'way that
it
serves
asa
protection
from tidal
waves.
It
should
be remembered,
,'
however,
that. all
seawalls
and bunds will
be substantially
damaged.
And in
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areas where they are built
right
on the coast,
they may be washed away
entirely.
This does not mean that
they are not effective,
because most of
the damage to a seawall
occurs
not with
the initial
impact of the wave but
rather
from the debris
which is carried
along on the bottom and scrapes
away at the wall
after
the leading
edge of the tidal
wave has passed.
Damage is also
caused by the tremendous
amount of debris
which is carried
along when the tidal
wave rushes back to the sea after
the cyclone
has
passed.
17.
Why did the tidal
wave in Andhra Pradesh appear
to be on fire?
There are two possible
explanations
for
this
phenomenon.
The first
is
that
there
is a tremendous
amount of phosphcrus
carried
out to sea by the
Krishna
River.
These small
particles
of phosphorus
lie
not only
in the
mud along
the river
banks but are also carried
out and depcsited
along
the
sea bed.
When the tidal
wave rushed
inland,
a large
amount of the surface
of the sea bed was picked
up in the turbulence
aqd carried
in the front
part
or leading
edge of the wave.
This phosphorus
may have been illuminated
by static
electricity
generated
by the wave as it moved inland.
T&is would
give the leading
edge of the wave the appearance
of being on fire.
The second explanation
is that,
as a tidal
wave moves, it
pushes a
large
amount of air
in front
of it.
This air
is very moist,
due to the rain
and the water
spray
immediately
in front
of the wave.
When this
meets with
the low pressure
air , a charge of static
electricity
can be generated
and
often
causes a phenomenon know-n as "St.
Elmo's
Fire",
This is simply
a glow
caused by the static
electricity
as it
is blown against
the wave and the spray
created
by the wave.
FCC:jwp
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